Tapered mode selective absorber for use in high power waveguide systems



Nov. 24, 1970 J. P. QUINE ETAL 3,543,199

TAPERED MODE SELECTIVE ABSORBER FOR USEIN HIGH POWER WAVEGUIDE SYSTEMS Flled Oct 3, 1968 I MA ,4;

United States Patent O US. Cl. 333-98 2 Claims ABSTRACT OF THE DISCLOSURE The invention comprehends a waveguide component adapted to selectively absorb unwanted TE TM modes of electromagnetic wave energy propagating in high power oversized waveguide systems. A section of rectangular waveguide that tapers from oversized system Waveguide dimensions to standard waveguide dimensions is used to provide 180 phase shifted reflection of TE modes and short circuiting of TM modes, thereby effecting maximum coupling thereof. The composite TE TM modes thus combined are then coupled out of the system by means of appropriate transverse side wall slots disposed in a section of rectangular waveguide attached to the larger end of the tapered section.

BACKGROUND OF THE INVENTION This invention relates to high power oversized waveguide systems and, in particular, to a waveguide mode selective absorber that is capable of selectively absorbing unwanted TE TM, modes of electromagnetic wave energy propagating therein.

Mode absorbers are sometimes required in oversize waveguide systems in order to damp out spurious mode resonances. Resonances can occur when a spurious mode becomes trapped within regions of the waveguide system having waveguide cross-sectional dimensions which are larger than in the surrounding regions. Trapping can occur, for example, in the region between two tapered transitions.

Mode absorbers are also required in order to prevent the radiation of spurious mode power from antennas connected directly to the output of an oversize waveguide system, that is, without the use of tapers. In this case, the mode absorber must provide high enough one-way loss to reduce the radiated spurious mode power to acceptable levels; this value of one-way loss may be considerably higher than that required to suppress resonance buildup in systems in which spurious mode trapping occurs, and may be obtained only with mode absorbers having inconveniently long lengths.

When a spurious mode experiences resonances as a result of being trapped, large amounts of power can be converted from the desired mode to the spurious mode. If a mode absorber is not employed, the converted power is dissipated mostly in the waveguide walls within the trapped region; the fields in this case can build up to high values, and overheating and dielectric breakdown can occur.

The principal problem encountered in the design of mode absorbers for ultra-high power waveguide systems is that of producing a required amount of absorption for the spurious mode, with only a negligible loss to the ultra-high power desired mode. The fractional power loss experienced by the desired mode must be very small, if the power dissipated in the mode absorber is to be kept reasonably low.

Effective practical absorption of TE,,,,,, TM degenerate mode pairs has been a long standing problem in the design of high power waveguides. A particular combination of these modes has zero longitudinal current all along the side walls, and therefore can not always be selectively absorbed by transverse slots on the side walls. To date, investigations have shown that the degeneracy between the TE and T M modes that occurs in rectangular waveguides can be removed by transforming the rectangular waveguide into an hexagonal shape. With the degeneracy removed, no combination of modes results in zero longitudinal sidewall currents, and transverse slots on the side walls can provide mode selective absorption.

These principles have been applied in the current state of the art by the utilization of a hexagonal mode absorber which consists of a length of hexagonal waveguide with slots placed along the centers of the side wall and oriented in the transverse direction. The length of the slots and the slot spacing of such a device are adjusted in order to obtain an optimum conductance per unit length. Because the longitudinal current for the desired TE mode is zero along the center of the side walls of the hexagonal waveguide, the transverse slots do not introduce loss to this mode. With this configuration, the TE TM composite modes having odd n can be absorbed; however, the modes having even it cannot be absorbed, because the longitudinal current along the center of the side walls is zero for these modes. The utility of this type of mode absorber is still further diminished because it also requires an impractically long length of hexagonal waveguide.

Hexagonal mode absorbers having appended side wall apertured rectangular waveguide sections have also been proposed. These devices utilize relatively short hexagonal sections and also eifectively absorb modes having even n indices. However, hexagonal waveguides with transitions on each end to rectangular waveguide are capable of supporting ghost mode resonances. They also have a somewhat limited bandwidth capability. The present invention is directed toward overcoming these and other deficiencies prevalent in prior art mode suppression.

SUMMARY OF THE INVENTION The absorption of unwanted TE TM modes of electromagnetic wave energy in high power oversized waveguide systems is accomplished in accordance with the invention by employing a section of tapered waveguide and a section of side wall apertured rectangular waveguide of oversize cross-sectional dimensions. It has been discovered that the differential phase between the TE and TM reflection coefficient produced when the T M and TE modes propagate through a tapered section of waveguide is approximately if the taper is gradual and there are no sharp discontinuities near cutoff. It has also been analytically determined that maximum coupling between the composite modes occurs when the differential phase is 180. The principal concept of the invention, therefore, is to insert a tapered section of waveguide into the high power oversized waveguide system to achieve maximum 180 phase shift between the TE and TM modes in order to couple the composite modes out of the system by means of transverse side wall slots.

It is a principal object of the invention to provide a new and improved waveguide mode selective absorber for use in high power oversized waveguide systems.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that is more compact than prior art devices.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that has greater bandwidth than prior art devices.

It is another object of the invention to provide a waveguide mode selective absorber of the type described that is free from ghost mode problems.

These, together with other objects, advantages and features of the invention will become more apparent from the following detailed description when taken in conjunction with the illustrative embodiments of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an elevational view of one presently preferred embodiment of the invention;

FIG. 2 is a plan view of the embodiment illustrated in FIG. 1;

FIG. 3 is a sectional view of FIG. 1 taken at 33; and

FIG. 4 is a sectional view of FIG. 1 taken at 44.

DESCRIPTION OF THE PREFERRED EMBODIMENT The novel mode suppression of the present invention was conceived as a result of the discovery that coupling occurs between the top wall and side wall composite modes of electromagnetic wave energy in a high power waveguide system upon reflection from a taper from standard to oversized waveguide.

The coupling occurs because of the difference in the phase of the reflection coefficients of the component TE and TM modes comprising the composite modes. The phase difference is produced near the cutoff region of the taper, because the TE modes tend to see an open circuit upon reflection, whereas the TM modes tend to see a short circuit. As a result of the coupling a certain amount of power will be converted from the to wall to the side wall composite modes upon reflection from the taper, and the power converted to the side wall composite mode can thus be absorbed by means of transverse slots on the side walls. Since a substantial phase difference can be obtained over a broad frequency bandwidth, substantial amounts of mode absorption can be obtained over broad frequency bandwidths; the bandwidth for mode absorption being limited by the absorption bandwidth of the slots.

The electric field along the top Wall of an unslotted Waveguide is due entirely to the top wall composite mode, the electric field of the side wall composite mode being zero along the top wall. Furthermore, the composite top wall mode is a characteristic mode of the rectangular waveguide, and consists of the particular combination of TE and TM characteristic modes for which the condition given by x)TE x)TM is satisfied. In Equation 1 (E and (EQ are the x components of the transverse electric fields of the component TE and TM modes comprising the top wall composite mode; the x-direction is parallel to the top Wall. Equation '1 shows that the net electric field in the x-direction is zero for the composite top wall mode.

The ratio of the y-components of the electric fields of the component TE and TM modes comprising the composite top wall mode is represented by the equation where the y-direction is perpendicular to the top wall, a is the waveguide width, b is the height of the waveguide and In and n are mode indices.

Assuming that a top wall mode is incident on the lossless waveguide at z=0, where the z-direction is parallel to the longitudinal axis of the waveguide, the net electric field, E perpendicular to the top wall can be expressed where B and ,B are the propagation constants of the 4 TE and TM modes in the hexagonal waveguide. Using Equation 2 in Equation 3, there results (Z Z) 1 then may be represented as cos fiq sin E) (s) where A9: (l Tn""1 TM) If the ratio 'na W the inverse ratio should be used.

Equation 6 shows that A has a minimum value equal to 1.0 (0 db) when A0 is an even multiple of 1r, and that A is a maximum when A0 is equal to an odd multiple of r. The maximum value, A is given by na mb max. my

The value of A can be determined as a function of frequency and L from Equation 5 by noting that 2 fl o)' rrr (9) where A is the free space Wavelength and (AQ and (AJ are the cutofl wavelengths of the TE and TM modes, respectively, in the waveguide.

The value of differential phase, A0, between the TE and TM mode reflection coeflicients produced by the taper can be inserted directly into the Equation 5 to calculate the mode absorption. From this equation, it is seen that a value of A0 equal to leads to a maximum mode absorption given by Equation 7. Since the T-E modes tend to see an open circuit while the TM modes tend to see a short circuit upon reflection from the taper, it should be expected that the differential phase, A0, will tend to be approximately 180, and that this should be relatively independent of frequency.

Exact calculations to determine the differential phase are rather difiicult, since in general reflections occur all along the length of the tapered waveguide, and not only at the cross section where the mode experiences cutofi. However, most of the reflection occurs in the vicinity of the cutoff cross-section, and for this reason the departure of A0 from the ideal value of 180 will be small.

Referring now to FIGS. 1 through 4, there is illustrated thereby one presently preferred embodiment of the invention. Tapered waveguide section 1 is connected to standard sized waveguide 4 by means of flanges 5 and fastening means 6. A section of oversized Waveguide 3 having transverse side wall slots 7 is connected between the larger end of tapered waveguide section 1 and oversized waveguide 2 also by means of flanges 5 and fastening means 6. The dimensions of the waveguide sections and the size and arrangement of side wall slots 7 are,

of course, determined by the electromagnetic wave frequency for which the system is designed and by the foregoing equations.

While the invention has been described in one presently preferred embodiment, it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In combination with a high power waveguide system having rectangular waveguide of oversized crosssectional dimensions and rectangular waveguide of standard cross-sectional dimensions, a mode selective absorber comprising a section of tapered rectangular waveguide, the larger end thereof having cross-sectional dimensions consistent with said waveguide of oversized cross-sectional dimensions and the smaller end thereof having dimensions consistent with and being connected to said waveguide of standard cross-sectional dimensions, said section of tapered rectangular waveguide having a taper that is operable to etfect substantially 180 phase shifted reflection of TE modes of electromagnetic wave energy and short circuiting of TM modes of electromagnetic wave energy propagating therethrough, and a section of rectangular waveguide of oversized cross-sectional dimensions having a plurality of transverse side wall slots therein, said last named section of rectangular waveguide being connected to said system waveguide having oversized cross-sectional dimensions and to the larger end of said section of tapered rectangular waveguide.

2. A mode selective absorber as defined in claim 1 wherein said plurality of transverse side wall slots are operable to couple TE TM composite modes of electromagnetic wave energy from the high power Waveguide system.

References Cited UNITED STATES PATENTS 2,938,179 5/1960 Unger 33334 3,050,701 8/1962 Tang 33334 3,218,586 11/1965 Khoury 33398 X HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R. 333-33, 73 

